The present invention relates to suspension devices and methods, and in particular to structures for mounting force-versus-displacement sensors, whereby external stress sources are isolated from active sensor components.
Accelerometers generally measure acceleration forces applied to a body by being mounted directly onto a surface of the accelerated body. One common type of accelerometer employs one or more force-versus-displacement or xe2x80x9cforce/displacementxe2x80x9d sensors for measurement of acceleration. Accelerometers employing two force/displacement sensors instead of the necessary minimum one sensor gain considerable advantage. If the two sensors operate in a push-pull mode, then many error sources such as thermally driven effects or drift may be rejected as common mode, while the difference signal represents the desired acceleration measurement. Occasionally, designs using two force/displacement sensors include two completely separate proof masses, which results in essentially two accelerometers, each having its own sensor, but operating in opposite directions. For numerous reasons, however, a two proof mass solution is not preferred. Rather, it is generally advantageous to have only one proof mass in an accelerometer. This preference for a single proof mass with two force/displacement sensors operating in a push-pull mode leads to an over-constrained system that results in considerable inherent errors.
A typical example of a prior art two sensor/single proof mass accelerometer, commonly referred to as a Tee design, is illustrated in FIGS. 1A and 1B. The accelerometer 10 illustrated in FIGS. 1A and 1B is a miniature structure fabricated from a substrate 12 of semiconductor material by conventional micromachining techniques. The substrate 12 is formed of a monocrystalline silicon material in a substantially planar structure, i.e., having substantially planar and parallel opposing offset upper and lower surfaces. The silicon substrate 12 often includes an upper silicon or active layer 14 that is electrically isolated from an underlying substrate 16 by an insulating layer 18, or an insulating layer is applied to active layer 14, as shown and described in U.S. Pat. No. 5,948,981, the entirety of which is incorporated herein by reference. The insulating layer 18 is may be a thin layer, e.g., about 0.1 to 10.0 micrometers, of an oxide, such as silicon oxide. The silicon substrate 12 is usually formed by oxidizing active layer 14 and underlying substrate 16, and adhering the two layers together. A portion of active layer 14 may be removed to bring the layer 14 to the desired thickness. The silicon oxide layer 18 retains its insulating properties over a wide temperature range to ensure effective mechanical resonator performance at high operating temperatures on the order of 100 degrees Celsius. In addition, the insulating layer 18 inhibits undesirable etching of the active layer 14 during manufacturing.
The accelerometer 10 includes an acceleration sensor mechanism 20 having one or more flexures 22 pliantly suspending a proof mass 24 from an inner sensor frame or plate 26 for movement of the proof mass 24 along an input axis I normal to the proof mass 24. The flexures 22 are preferably etched near or at the center of the underlying substrate 16, i.e., substantially centered between the opposing upper and lower surfaces of the underlying substrate 16. Optionally, the flexures 22 are formed by anistropically etching in a suitable etchant, such as potassium hydroxide (KOH). The flexures 22 define a hinge axis H about which the proof mass 24 moves in response to an applied force, such as the acceleration of the accelerated body, for example, a vehicle, aircraft or other moving body having the accelerometer 10 mounted thereon. The sensor mechanism 20 includes a pair of force/displacement sensors 28 coupled between the proof mass 24 and the sensor frame 26 for measuring forces applied to the proof mass 24. The force/displacement sensors 28 are, for example, mechanical resonators formed from the active silicon layer 14 as double-ended tuning fork (DETF) force sensors. A known oscillator circuit, shown and described in above-incorporated U.S. Pat. No. 5,948,981, drives the mechanical resonators 28 at their resonance frequency. In response to an applied force, the proof mass 24 rotates about the hinge axis H, causing axial forces, either compressive or tensile, to be applied to the mechanical resonators 28. The axial forces change the frequency of vibration of the mechanical resonators 28, and the magnitude of this change serves as a measure of the applied force or acceleration. In other words, the force/displacement sensors 28 measure the applied acceleration force as a function of the displacement of the proof mass 24.
Undesirable external stresses and strains may be induced in the sensitive acceleration sensor mechanism 20 by, for example, mechanical coupling of the accelerometer sensor frame 26 to a silicon cover plate 30 which in turn is typically connected to a ceramic or metal mounting plate 32. Many methods are known for isolating the sensitive acceleration sensor mechanism 20 from such undesirable stresses and strains. Typically, the sensor frame 26 is suspended from a second outer or external frame portion 34 by flexures 36 formed by overlapping slots 38 and 40 through the substrate 12. The sensor frame 26 is thus able to move relative to the outer frame 34, as shown and described in U.S. Pat. No. 5,948,981, which is incorporated herein. Such isolation minimizes the distortion of the sensor frame 26, and thereby decreases the effects of external stresses and strains on the mechanical resonators 28.
FIG. 1B is a cross-section view taken through the accelerometer 10 along the resonators 28. As discussed above and shown in FIG. 1B, the proof mass 24 is free to rotate about the flexures 22 when subjected to acceleration along the input axis I according to the principle of Newton""s law, F=ma. This rotation is constrained by the action of two force/displacement sensors 28, shown as DETF resonators, positioned on a surface of the mechanism as shown. These two vibrating beam force sensors 28 provide push-pull variable frequency output signals since, when the proof mass 24 is displaced relative to the plane of the sensor mechanism 20, one DETF resonator 28 is under compression while the other is under tension. The difference between the two frequencies represents the measured acceleration. Common mode frequency shifts, on the other hand, are rejected as errors driven by unwanted sources such as temperature, mechanism stress, or drift.
FIGS. 1A and 1B also illustrate the common over-constraint problem that arises due to the single proof mass 24 being constrained by two or more elements, in this case DETF resonators 28. The two DETF resonators 28 constrain not only the proof mass 24 common to each, but also impact each other through the common proof mass 24. Thus, any strains occurring in the sensor frame 26 are transmitted not only to the proof mass 24, but through the proof mass 24 to the other DETF resonator 28. Since the only significant compliance in the system is the sensing DETF resonators 28 themselves, almost the entire strain appears as an error output from the DETF resonators 28. Thus, undesirable errors are generated in the DETF resonators 28 from inputs having nothing to do with the acceleration being measured. These errors can be quite large since the compliance through the DETF resonators 28 must be low to detect acceleration with sufficient accuracy to be useful in practical systems.
FIG. 2 illustrates an accelerometer 40 having a common offset design of the prior art wherein the DETF resonators 28 are offset on either side of a proof mass 42 such that the two sensors operate in the push-pull mode described above. The offset DETF resonators 28 again constrain not only the proof mass 42 common to each, but also impact each other through the common proof mass 42. Furthermore, the offset DETF resonators 28 are again the only significant compliance in the system so that any strains occurring in the sensor frame 44 are transmitted to the proof mass 42 and through the proof mass 42 to the other DETF resonator 28, and almost the entire strain appears as an error output from the DETF resonators 28.
The present invention provides an apparatus and method that minimizes the over-constraint errors by providing an additional degree of freedom in the system, in contrast to the prior art devices and methods. The apparatus and method of the present invention thus provide improved performance from an accelerometer utilizing multiple force/displacement sensors in combination with a single proof mass. Since current known micromachining techniques can effectively produce the invention features in a substrate simultaneously with other accelerometer features, this improved performance comes at essentially no additional cost.
The apparatus and method of the present invention provides a suspension structure for suspending one or more force-versus-displacement sensors for measuring displacement of a pendular structure relative to a frame structure. The suspension structure includes a frame structure and a pendular structure, the pendular structure having a base structure suspended from the frame structure for rotation about a first axis, a beam structure spaced away from the first axis, and a flexure suspending the beam structure from the base structure for rotation about a second axis substantially perpendicular to the first axis.
According to one aspect of the invention, the flexure suspending the beam structure from the base structure further constrains the beam structure and substantially restrains the beam structure from rotation out of plane with the first axis.
According to another aspect of the invention, a center of mass of the beam structure is substantially colocated with the second axis of rotation.
According to another aspect of the invention, the beam structure includes mounting positions for force-versus-displacement sensors. The mounting positions are located at opposite ends of the beam structure in substantial alignment with a center of mass of the beam structure and offset from the first axis of rotation.
According to yet another aspect of the invention, the frame structure of the invention is embodied as an accelerometer, wherein force-versus-displacement sensors are coupled between the frame structure and respective positions on the beam structure that are located on opposite sides of the flexure that suspends the beam structure from the base structure.
According to still other aspects of the invention, a method is provided for resolving nonlinearities in an accelerometer, the method being formed of suspending a base portion of a proof mass for rotation about a first axis relative to a frame member; suspending a beam portion of a proof mass for rotation about a second axis relative to the base portion of the proof mass; and suspending a force-versus-displacement sensor between different positions on the frame member and each of two positions on the beam portion that are spaced apart on opposite sides of the second axis of rotation.
According to another aspect of the method of the invention, suspending a beam portion of a proof mass includes suspending the beam portion for rotation about a second axis that is substantially perpendicular to a plane containing the first axis.
Alternatively, the beam portion is further structured such that a center of mass thereof is substantially aligned with the second axis about which the beam portion rotates with respect to the base portion of the proof mass.
According to another aspect of the method of the invention, suspending a force-versus-displacement sensor includes suspending the force-versus-displacement sensor between the frame member and opposite ends of the beam portion.
According to still another aspect of the method of the invention, suspending a force-versus-displacement sensor alternatively includes suspending the force-versus-displacement sensor between the frame member and termination points on the beam portion that are substantially aligned with the second axis of rotation about which the beam portion rotates with respect to the base portion of the proof mass.